fix: Add input validation to model load

[mattwittwer]

What does the PR do?

Simplify the model load command by invoking the python backend stub without shell interpretation. Additionally add checks for invalid characters in provided model names

Checklist

• PR title reflects the change and is of format <commit_type>: <Title>
• Changes are described in the pull request.
• Related issues are referenced.
• Populated github labels field
• Added test plan and verified test passes.
• Verified that the PR passes existing CI.
• Verified copyright is correct on all changed files.
• Added succinct git squash message before merging ref.
• All template sections are filled out.
• Optional: Additional screenshots for behavior/output changes with before/after.

Commit Type:

Check the conventional commit type
box here and add the label to the github PR.

• build
• ci
• docs
• feat
• fix
• perf
• refactor
• revert
• style
• test

Related PRs:

Tests for these changes have been added here: triton-inference-server/server#8276

Where should the reviewer start?

The majority of the changes are in the src/stub_launcher.cc
Review the updated flow for building the model load command.

Test plan:

Added testing to: triton-inference-server/server#8276

• CI Pipeline ID:

31707450

Caveats:

Background

Related Issues: (use one of the action keywords Closes / Fixes / Resolves / Relates to)

• closes GitHub issue: #xxx